A concept for recording rolled fingerprints was described for the first time in U.S. Pat. No. 4,553,837 A. In this case, a device for image capture moves around the finger. This concept was optimized (U.S. Pat. No. 4,783,167 A) and alternatives with units for image capture which are likewise moved were disclosed in U.S. Pat. Nos. 4,787,742 A and 4,946,276 A.
The above-mentioned concepts have the advantage over mechanical rolling of a finger on paper that the finger need not be moved during the rolling process. With any type of rolling movement, a high-quality image capture is difficult and prone to error. Some of the image information can be invalidated and rendered unusable or irrecoverable because of too much pressure, too little pressure, slippage, incorrect or fluctuating rolling speed or changes in direction during rolling. On the other hand, arrangements such as those indicated above with moving components are maintenance-intensive and prone to equipment failure.
Further, concepts have been presented in which the finger is rolled on a support surface provided for this purpose. In this case, images are captured continuously through this support surface and suitably put together (U.S. Pat. No. 4,933,976 A). The algorithms for image composition and also the apparatus for image capture have been constantly improved (U.S. Pat. No. 6,597,802 B1, U.S. Pat. No. 6,795,570 B1, U.S. Pat. No. 7,095,880 B2, U.S. Pat. No. 7,613,334 B2). For example, a method for reducing smearing resulting from slippage during rolling was developed in U.S. Pat. No. 5,748,766 A.
There is a multitude of demands with respect to the quality, resolution and fidelity to the original of the recorded images of skin textures. Thanks in no small part to strict requirements on the part of organizations working in the field of police records such as the Federal Bureau of Investigation (FBI), for example, there is a high degree of standardization in identification systems in order to ensure the highest possible certainty of identification on the one hand and, on the other hand, to allow comparison between datasets which have been recorded by different systems. For example, such systems must have a resolution of at least 500 ppi corresponding to an average sensor element spacing of 50.8 μm. Further, contrast transfer function (CTF), signal-to-noise ratio (SNR) and distortion must comply with certain requirements. Finally, the gray value scale must include at least 200 values and the image field must be illuminated as homogeneously as possible both in the near pixel environment and in the image overall.
Currently, the optical arrangements chiefly used for acquiring rolled fingerprints which meet the high quality requirements mentioned above work on the principle of frustrated total internal reflection (FTIR—also often referred to merely as TIR). In commercially available equipment, the user is guided by information which is presented external to the support surface, e.g., by a screen mounted next to it. Accordingly, during the recording of roll fingerprints the person must always glance back and forth between the support surface and the screen.
Equipment operating by TIR has been known for decades. Its optical components and imaging beam path have been continually miniaturized as is known, e.g., from U.S. Pat. No. 7,379,570 B2.
A concept for displaying information about the scanning results or the influence of the user for improving the scan is not known in the TIR concept in connection with conventional optical imaging and is difficult to implement technologically because the display components must not be allowed to obstruct the optical beam path.
In a further development, it was attempted through alternative optical concepts without conventional imaging, i.e., without optical mirrors and/or lenses, to combine the advantage of the high image quality of the TIR principle with a compact construction. Approaches for this purpose are described in US 2012/0321149 A1. Another optical concept for a flat construction without imaging optics is described in U.S. Pat. No. 7,366,331 B2.
Besides optical sensors, capacitive sensors are also known. Recording can be implemented with different concepts in this case as is disclosed, e.g., in U.S. Pat. No. 5,325,442 A, DE 197 56 560 A1, U.S. Pat. Nos. 6,437,583 B1, 6,016,355 A and 5,956,415 A. All have in common that the capacitance of the skin is measured by way of a reference measurement at a defined capacitor, read out electronically and converted into a digital image via an analog-to-digital converter. Real-time displays of the results with instructions for correction for the user are difficult to implement.
Further, concepts also exist for acoustic recording (e.g., U.S. Pat. No. 8,601,876 B2) and piezo-electric recording (e.g., U.S. Pat. No. 4,394,773 A) of fingerprints, but they haven't been used for recording rolled fingerprints because of their resolution features and quality features.
All of the above-mentioned optical concepts without conventional imaging as well as the non-optical concepts enable layer structures of small constructional size. They are suitable for detecting flat fingerprints as well as rolled fingerprints. Thus far, however, none of these disclosed concepts allows the display of user instructions for recording a rolled fingerprint with quality control and instructions for correction directly on the support surface.
In spite of software algorithms which have been greatly improved in the interim for optimizing the individual recordings and for putting the individual images together in a suitable manner, the rolling process is still prone to error and the results of the put-together roll images often do not conform to the desired quality.
Apart from hardware of sufficient quality for image capture and software for calculating and optimizing the roll image, it is enormously important that the rolling process proceeds successfully. This means that the initial and final positioning of the finger is correct and that the rolling speed, the pressure with which the finger is pressed on the support surface and the rolling direction are as consistent as possible and are adapted to the image capture system. The optimization of the rolling process can be carried out, for example, by specially trained personnel, but this is very time-consuming and costly and is not always or everywhere possible. Therefore, it would be desirable if the equipment itself were to communicate or interact with the user in such a way that the user is guided and instructed during the rolling process so that an optimal roll image of the finger is made. Even if the equipment were used by trained personnel, a guided rolling process would represent a simplification and assistance for the personnel, e.g., for personnel who only make unevenly rolled fingerprint recordings and who, for lack of experience, would always make use of any instruction in order to prevent operating errors.
U.S. Pat. No. 9,202,100 B2 describes a concept for guiding the user in which the user is shown information about quantity and position, steadiness and correct pressing pressure of the finger via a separate display in the vicinity of the support surface. The purpose of this is primarily to inform the user of errors in the placement of single-finger prints and four-fingerprints. The concept is characterized in that a two-dimensional display unit is arranged next to the recording unit. It describes the recording of flat, i.e., not rolled, fingerprints. The object of the user guidance shown herein is the display of an image sequence in which a view of the backside of the user's positioned extremity (finger or hand) is simulated for the user. In this case, a transition from an existing error state to a desired final state is shown by means of symbolic depiction of fingers or hands. In so doing, no characters are used and no print result image is shown. It is disadvantageous that when the user is prompted to perform an action this action cannot be controlled or influenced during the process. However, this would be useful or even necessary when recording rolled fingers.